A liquid crystal display mode which has meanwhile found widespread interest and commercial use is the so-called PS (“polymer sustained”) or PSA (“polymer sustained alignment”) mode, for which the term “polymer stabilised” is also occasionally used. In PSA displays an LC medium is used that contains an LC mixture (hereinafter also referred to as “host mixture”) and a small amount, for example 0.3% by weight and typically <1% by weight, of one or more polymerisable compounds, preferably polymerisable monomeric compounds. After filling the LC medium into the display, the polymerisable compounds are polymerised or crosslinked in situ, usually by UV photopolymerisation, optionally while a voltage is applied to the electrodes of the display. The polymerisation is carried out at a temperature where the LC medium exhibits a liquid-crystal phase, usually at room temperature. The addition of polymerisable mesogenic or liquid-crystalline compounds, also known as reactive mesogens or “RMs”, to the LC host mixture has proven particularly suitable.
The PS(A) mode is meanwhile used in various conventional LC display types. Thus, for example, PS-VA (“vertically aligned”), PS-OCB (“optically compensated bend”), PS-IPS (“in-plane switching”), PS-FFS (“fringe-field switching”), PS-UB-FFS (“Ultra Brightness FFS) and PS-TN (“twisted nematic”) displays are known. The polymerisation of the RMs preferably takes place with an applied voltage in the case of PS-VA and PS-OCB displays, and with or without, preferably without, an applied voltage in the case of PS-IPS displays. As a result a pretilt angle of the LC molecules is generated in the display cell. In case of PS-OCB displays, for example, it is possible for the bend structure to be stabilised so that an offset voltage is unnecessary or can be reduced. In case of PS-VA displays, the pretilt has a positive effect on the response times. For PS-VA displays, a standard MVA (“multidomain VA”) or PVA (“patterned VA”) pixel and electrode layout can be used. It is also possible to use only one structured electrode without protrusions, which significantly simplifies production and improves contrast and transparency.
Furthermore, the so-called posi-VA mode (“positive VA”) has proven to be particularly suitable. Like in conventional VA and PS-VA displays, the initial orientation of the LC molecules in posi-VA displays is homeotropic, i.e. substantially perpendicular to the substrates, in the initial state when no voltage is applied. However, in contrast to conventional VA and PS-VA displays, in posi-VA displays LC media with positive dielectric anisotropy are used. Like in IPS and PS-IPS displays, the two electrodes in posi-VA displays are arranged only on one of the two substrates, and preferably exhibit intermeshed, comb-shaped (interdigital) structures. Upon application of a voltage to the interdigital electrodes, which create an electrical field that is substantially parallel to the layer of the LC medium, the LC molecules are switched to an orientation substantially parallel to the substrates. In posi-VA displays, a polymer stabilisation by addition of RMs to the LC medium, which are then polymerised in the display, has also proven to be advantageous. Thereby a significant reduction of the switching times can be achieved.
PS-VA displays are described for example in EP1170626 A2, U.S. Pat. Nos. 6,861,107, 7,169,449, US 2004/0191428A1, US2006/0066793A1 and US2006/0103804A1. PS-OCB displays are described for example in T.-J-Chen et al., Jpn. J. Appl. Phys. 45, 2006, 2702-2704 and S. H. Kim, L.-C-Chien, Jpn. J. Appl. Phys. 43, 2004, 7643-7647. PS-IPS displays are described for example in U.S. Pat. No. 6,177,972 and Appl. Phys. Lett. 1999, 75(21), 3264. PS-TN displays are described for example in Optics Express 2004, 12(7), 1221.
PSA displays can be operated as either active-matrix or passive-matrix displays. In case of active-matrix displays individual pixels are usually addressed by integrated, non-linear active elements like for example transistors (such as thin-film transistors or “TFTs”), whereas in passive-matrix displays individual pixels are usually addressed by the multiplex method as known from prior art.
A PSA display may also comprise an alignment layer on one or both of the substrates forming the display cell. The alignment layer is usually applied on the electrodes (in case such electrodes are present) such that it is in contact with the LC medium and induces initial alignment of the LC molecules. The alignment layer may comprise or consist of, for example, a polyimide, which may also be rubbed or prepared by a photoalignment method.
In particular for monitor and especially TV applications optimisation of the response times, but also of the contrast and luminance (and thus transmission) of the LC display is still desired. The PSA method can provide significant advantages here. Especially in case of PS-VA, PS-IPS, PS-FFS and PS-posi-VA displays, a shortening of the response times, which correlate with a measurable pretilt in test cells, can be achieved without significant adverse effects on other parameters.
Prior art has suggested biphenyl diacrylates or dimethacrylates, which are optionally fluorinated, as polymerisable compounds for use in PSA displays.
However, the problem arises that not all combinations of LC host mixture and polymerisable compounds are suitable for use in PSA displays because, for example, only inadequate tilt angles or no tilt angles at all could be generated or because, for example, the voltage holding ratio (VHR) is inadequate for TFT display applications. In addition, it has been found that the LC mixtures and polymerisable compounds known from prior art still have some disadvantages when used in PSA displays. Thus, not every known polymerisable compound which is soluble in the LC host mixture is suitable for use in PSA displays. In addition, it is often difficult to find a suitable selection criterion for the polymerisable compound besides direct measurement of the pretilt in the PSA display. The choice of suitable polymerisable compounds becomes even smaller if UV photopolymerisation without the addition of photoinitiators is desired, which is advantageous for certain applications.
In addition, the selected combination of LC host mixture/polymerisable compound should have a low rotational viscosity and good electrical properties, in particular a high VHR. In PSA displays, a high VHR after irradiation with UV light is particularly important, because UV exposure does not only occur as normal exposure during operation of the finished display, but is also a necessary part of the display production process.
In particular, it is desirable to have available improved materials for PSA displays which produce a particularly small pretilt angle. Preferred materials are those which, compared to prior art materials, can generate a lower pretilt angle after the same exposure time, and/or can generate at least the same pretilt angle after a shorter exposure time. This would allow reducing the display production time (“tact time”) and production costs.
Another problem observed in prior art is that LC media for use in PSA displays, including but not limited to displays of the PSA type, do often exhibit high viscosities and, as a consequence, high switching times. In order to reduce the viscosity and response time of the LC medium, it has been suggested in prior art to add LC compounds with an alkenyl group. However, it was observed that LC media containing alkenyl compounds often show a decrease of the reliability and stability, and a decrease of the VHR especially after exposure to UV radiation. Especially for use in PSA displays this is a considerable disadvantage, because the photopolymerisation of the polymerisable compounds in the PSA display is usually carried out by exposure to UV radiation, which may cause a VHR drop in the LC medium.
Another problem to be solved is that the RMs of prior art, which are used as polymerised compounds in PSA displays, do often have high melting points, and do only show limited solubility in many commonly used LC mixtures. As a result these RMs tend to spontaneously crystallise out of the LC mixture. In addition, the risk of spontaneous polymerisation prevents that the LC host mixture can be warmed in order to better dissolve the RMs, so that a high solubility even at room temperature is required. In addition, there is a risk of phase separation, for example when filling the LC medium into the LC display (chromatography effect), which may greatly impair the homogeneity of the display. This is further aggravated by the fact that the LC media are usually filled in the display at low temperatures in order to reduce the risk of spontaneous polymerisation (see above), which in turn has an adverse effect on the solubility.
Another problem in the production of PSA displays is the presence and removal of residual amounts of monomers which did not polymerise during the polymerisation step that is necessary for generation of the pretilt angle in the display. These unpolymerised monomers may adversely affect the properties of the display, for example by polymerising in an uncontrolled manner during display operation.
Thus, the PSA displays known from prior art often exhibit the undesired effect of so-called “image sticking” or “image burn”, i.e. the image produced in the LC display by temporary addressing of individual pixels still remains visible even after the electric field in these pixels has been switched off, or after other pixels have been addressed.
Image sticking can occur for example if LC host mixtures having a low VHR are used. The UV component of daylight or the display backlight can cause undesired decomposition reactions of the LC molecules and initiate the production of ionic or free-radical impurities. These can accumulate in particular at the electrodes or the alignment layers, where they reduce the effective applied voltage. This effect can also be observed in conventional LC displays without a polymer component.
An additional image sticking effect caused by the presence of unpolymerised monomers is often observed in PSA displays. Uncontrolled polymerisation of residual monomers can be initiated by UV light from the environment or the display backlight. In the switched display areas, this can change the tilt angle after a number of addressing cycles. As a result, a change in transmission in the switched areas may occur, while transmission remains unchanged in the non-switched areas.
During production of the PSA display it is therefore desirable that polymerisation of the monomers proceeds as completely as possible and the presence of unpolymerised monomers in the display can be excluded or reduced to a minimum. Thus, polymerisable monomers and LC host mixtures are required which enable or support quick and complete polymerisation. In addition, a controlled reaction of the residual monomer amounts is desirable. This could be achieved by providing improved polymerisable compounds that polymerise quicker and more effectively than the materials of prior art.
A further problem that has been observed in PSA displays is an un-sufficient pretilt angle stability. Thus, it was observed that the generated pretilt angle in the display does not remain constant but can deteriorate after the display was subjected to voltage stress during display operation. This can negatively affect the display performance, e.g. by increasing the black state transmission and hence lowering the contrast.
Another problem observed in prior art is that the use of conventional LC media in LC displays, including but not limited to displays of the PSA type, often leads to the occurrence of mura in the display, especially when the LC medium is filled in the display by using the one drop filling (ODF) method. This phenomenon is also known as “ODF mura”. It is therefore desirable to provide LC media which lead to reduced ODF mura.
Another problem observed in LC displays, including but not limited to PSA type displays, is the so-called “frame mura”. This may occur at the edges of the display cell where the LC medium is in contact with the sealant material which is used to connect the two substrates of the display cell and seal the edges, and which can cause orientation defects in the LC layer. The sealant material typically contains acrylates that are cured by UV photopolymerisation using appropriate photoinitiators after the LC medium has been filled into the display cell. Therefore, in PSA displays there is also the problem that the polymerisable compounds contained in the LC medium can show undesired pre-polymerisation during the curing process of the sealant material.
In order to suppress undesired pre-polymerisation of the polymerisable compounds in LC media for PSA displays, it is possible to add small molecule inhibitors to the LC medium. However, this can lead to undesired impurities in the display.
There is thus still a great demand for PSA displays and LC media and polymerisable compounds for use in such displays, which have improved properties and can help to avoid the drawbacks as described above completely or at least partially.
In particular, it is desirable to provide improved PSA displays and LC media, and LC host mixtures, polymerisable compounds or components used therein, wherein the polymerisable compounds should have low melting points and high solubility in the LC host mixture, and wherein the LC media and PSA displays show one or more of the following improvements: High specific resistance, high VHR values and high reliability especially after UV and/or heat exposure, low threshold voltage, large working-temperature range, low viscosity, short response times even at low temperatures, high birefringence, good UV absorption especially at longer wavelengths, quick and complete polymerisation of the polymerisable compounds, reduced undesired pre-polymerisation of the polymerisable compounds, quick generation of a low pretilt angle, high stability of the pretilt angle even after long operation time and after UV or heat exposure, reduced image sticking, reduced ODF mura and reduced frame mura in the display, a multiplicity of grey shades, high contrast and a broad viewing angle.
The present invention is based on the object of providing novel suitable materials, in particular polymerisable compounds or components, LC host mixtures, and LC media comprising the same, for use in PSA displays, which do not have the disadvantages and drawbacks observed in the LC media and displays of prior art as described above, and which show one or more of the above mentioned desired improvements.
The present invention is also based on the object of providing LC media for use in PSA displays, which enable high specific resistance values, low viscosity and high VHR while enabling quick and complete polymerisation of the polymerisable compounds.
A further object of the invention is the provision of novel and improved polymerisable compounds, which are in particular suitable for optical, electro-optical and electronic applications, and of suitable synthesis processes and intermediates for the preparation of the novel polymerisable compounds.
The above objects have been achieved in accordance with the present invention by materials and processes as described and claimed hereinafter.
It has surprisingly been found that at least some of the above-mentioned problems can be solved, and one or more of the above-mentioned improvements can be achieved, by using polymerisable compounds and/or LC media as disclosed and claimed hereinafter, especially by using an LC medium which comprises an LC host mixture and a polymerisable component, wherein the polymerisable component comprises or consists of one or more compounds of formula I as described below, and preferably comprises one or more RMs.
The compounds of formula I are characterized in that they contain a stabilizing functional group that is capable of catching free radicals, and additionally contain one or more polymerizable functional groups. The stabilising group is for example 2,6-di-t-butylphenol, or a hindered amine light stabiliser (HALS) group, such as 2,2,6,6-tetraalkylpiperidinyl-oxy (NO-HALS), 2,2,6,6-tetraalkylpiperidinyl-hydroxy or 2,2,6,6-tetraalkylpiperidinyl. Compared to non-polymerizable stabilizers as known from prior art, the polymerizable stabilizers of formula I provide advantages like delayed polymerization, higher reliability and higher VHR values after the PSA process.
It was surprisingly found that the use of a polymerisable component and an LC medium comprising it, as claimed and described hereinafter, in PSA displays allows good control of the polymerisation and tilt generation process. Thus, it was found that the use of a compound of formula I, optionally together with a conventional RM, leads to a delay of the polymerisation and pretilt generation process, because the compound of formula I acts as an inhibitor that catches free radicals which start the polymerisation process. After the delay phase, polymerisation and tilt generation proceed without significant negative influence on the desired low pretilt and low amount of residual monomer finally achieved, which are comparable to an LC medium without the stabilising compound of formula I.
The use of a polymerisable component and an LC medium comprising it as described and claimed hereinafter in PSA displays also allows to suppress undesired, process-induced pre-polymerisation, for example during the curing process of the sealant material, which is expected to reduce frame mura.
The polymerisable component and LC medium of the present invention also enable a greater flexibility in varying the process conditions and broadening the process window, for example with respect to monomer concentration, LC host mixture composition, UV irradiation intensity, irradiation time, lamp spectrum, temperature, voltage or alignment layer material.
In addition, the compounds of formula I can also be used to replace small molecule stabilisers that are often added to the LC medium to prevent undesired spontaneous polymerisation of the polymerisable component for example during storage or transport.
Finally, the use of the polymerisable component and LC medium according to the present invention in PSA displays allows a complete UV-photopolymerisation reaction, controlled generation of large and stable pretilt angles, reduces image sticking and mura in the display, enables high VHR values after UV photopolymerisation, especially in case of LC host mixtures containing LC compounds with an alkenyl group, and enables to achieve fast response times, a low threshold voltage and a high birefringence.
US2005/0192419A1 discloses 2,2,6,6-tetramethylpiperidinyloxyacrylate and 2,2,6,6-tetramethylpiperidinyloxymethacrylate and their use in scorch-retardant compositions, but does neither disclose nor suggest the use of these compounds in LC media or PSA displays.